Procalcitonin (PCT) is a prohormone of calcitonin produced physiologically by C-cells of the thyroid gland. PCT, a polypeptide consisting of 116 amino acids and with a molecular weight of 13 kDa is synthesized by extrathyroidal tissues during pathological processes. This protein plays a key role in the systemic inflammatory response syndrome caused by bacterial, fungal and protozoal infections. It behaves as an acute phase protein. High serum procalcitonin levels correlate with severe systemic infections. Resent studies have led to the conclusion that PCT may be useful in the diagnosis, treatment and prognosis of patients with infections. The assay of this biochemical parameter is made in many different diseases including systemic inflammatory rheumatic diseases. Elevation of PCT is useful in differentiation of infectious versus non-infectious fever in rheumatic diseases. PCT behaves differently to other markers of inflammatory response in the case of fever caused by disease activity in patients with inflammatory rheumatic diseases.